In a message dated 970922 00:00:04 EDT, robert@woozy.com writes:
> I know
> that this throws the speedo off, but in which direction? Am I going faster
> than the speedo says or slower? Im guessing slower, but math was never my
> strongest area.
Robert:
Use the formulas below to figure the correct speed for a given rpm (all
example calculations assume a 195/70 R14 tire on a car with a 3.91 rear axle
ratio):
Tire diameter: (2 X tire size X aspect ratio)/25.4 + wheel diameter:
(2 X 195 X 0.7)/25.4 + 14 = 24.7"
MPH/1000RPM: (tire diameter X 2.975)/axle ratio
(24.7 X 2.975)/3.91 = 18.8 MPH
RPM @ 65MPH (65000 X axle ratio)/(2.975 X tire diameter)
(65000 X 3.91)/(2.975 X 24.7) = 3452
To determine speed at a different RPM, multiply by the appropriate factor,
ie, for 2500 RPM, multiply the results above by 2.5.
To find the RPM at a different speed, substitute the correct factor in the
formula above, ie, for 45 MPH, use 45000, instead of 65000.
If you want to calculate for an overdrive unit, multiply the axle ratio by
the OD ratio, and use that value in place of the axle ratio in the formulas.
The 2.975 in the equations takes into account the conversions from inches to
feet, feet to miles, RPM to MPH, and includes pi.
In general, changung the diameter of the tires has the same effect as
changing the rear end ratio. A larger diameter has the effect of changing to
a higher gear ratio (lower numerically), and the car is going faster than
indicated, and a smaller diameter has the effect of changing to a lower gera
ratio (higher numerically), and the car is going slower than imdicated. thus,
a slightly larger diameter would change the 3.91 in the above example to
something on the order of 3.85  3.90 ( I haven't done the math  these are
just relative numbers for illustration). Sometime in the near future, I will
do the math, and post a formula to arrive at the effective rear end ratio for
changes in tire diameter.
The above formulas are as accurate as the data fed into them, which,
unfortunately, aren't that accurate. There can be a variation in tire
dimensions between two different models of tires produced by the same
manufacturer, even though the tires have the same size designation. Not
really a problem, though, because your speedometer is not that accurate
either, nor is your tachometer. Also unfortunately, the radar guns used by
the police are quite accurate if used properly, and are considered by the
courts to be spot on!
Dan Masters,
Alcoa, TN
'71 TR63000mile/year driver, fully restored
'71 TR6undergoing full restoration and Ford 5.0 V8 insertion  see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT3000mile/year driver, original condition
'68 MGBGTorgan donor for the '74
